Protection of metal articles against corrosion by coating by electrodeposition



Patented July 29, 1941 PROTECTION OF METAL ARTICLES AGAINST CORROSION BYCOATING BY ELECTRO- DEPOSITION Samuel Thomas Roberts, Southgate, London,

England No Drawing. Application June 14, 1938, Serial No. 213,625. InGreat Britain July 24, 193':

6 Claims.

This invention relates to the protection of metal articles againstcorrosion, and its principal object is the provision on ferrous andother metal articles of an anti-corrosion surface alloy or coating ofgreat efficacy and durability.

According to the invention, I provide a process for protecting a metalarticle against corrosion comprising the electro-deposition of an'alloycon-' taining cadmium and mercury from an alkaline bath containingaluminium in solution 'in addition to the constituents of the alloy.

The aluminium appears to increase the throwing power of the bath to suchan extent that the recesses and interior surfaces of irregularlyshapedand hollow articles are found to be quite uniformly covered without anynecessity for special anode shapes or distributions to conform with theparticular article to be treated, assuming of course that the article iscompletely immersed and that no air pockets are allowed to form in it.In this connection, it is preferable to suspend the article as acathode, in the bath by means of aluminium carrier wires or hooks, andthese may with advantage be arranged to penetrate as far as possibleinto any deep cavities or interio spaces in the article.

The following is an example of a process according to the inventionwhich has given particularly good results:

Example I An aqueous solution is made up in a conventional way, ifnecessary with the aid of heat, containing per gallon of water:

Sodium cyanide ounces 12 Caustic soda .do 12 Zinc sulphate do 10.Cadmium sulphate do 4' Blchloride of mercury do 2 Potassium aluminiumsulphate do 3 Dextrine dn 2 Glycerhyzine (ammoniated glycerine) grams 1%Alternatively the anode may be cast from a homogeneous mixture oramalgam of zinc, cadmium and mercury.

Example II Ounces Sodium cyanide 12 Caustic soda 12 Cadmium sulphate 12Bichloride of mercury A; to 2 Aluminium potassium sulphate' 2 7 Cobaltsulphate M; to 2 Dextrine 2 Example III Sodium cyanide 12 Caustic soda12 Cadmium sulphate 12 Bichloride of mercury e to 2 Aluminium potassiumsulphate 2 1 Nickel sulphate FA; to 2 Dextrine 2 The constituentsspecified in Examples 1 and II are in each case, added to one gallon ofwater.

The cobalt or nickel sulphate is added to the so-=.

- Example I. The anode in this case may comprise zinc, cadmium andmercury in the ratio,8:4:1 or- 8:221, the nickel or cobalt being addedas a me tallic salt to the solution when required. Or the anode may.comprise cadmium and mercury in the ratio 9:1.

de-scaling (if necessary) is suspended-in the bath by means of wires orcarriers of aluminium. and

a current of 10 to 25 amperes per square foot of article surface ispassed. It is preferable to use the article as an anode for a fewmoments initially, as this opens up its surface pores and enables afirmly adherent coating or surface alloy iron articles a perfect andlasting resistance to oxidation and general corrosion. It has also beenobserved that the articles so treated do not Or I may employ a cadmiumanode carried'by a cobalt or nickel member. The article to be treated,after cleaning and have their dimensions appreciably increased thereby,this being of course a valuable feature where nuts, bolts and working orinterfitting parts are concerned. If the cathodic step is undulyprolonged, further metal is deposited, but in the form of a friablelayer or fur which may be readily brushed away from the underlyingsurface coating or alloy proper.

The use of mercury appears to give the surface alloy a remarkably dense,homogeneous and nonporous nature, despite its thinness, and itsincorporation in the anode not only ensures replenishment of the bath atthe correct rate but enables the zinc and cadmium to be alloyed with auniformity not otherwise possible. The anode thus wears away veryevenly, and the deposition as a whole therefore proceeds very uniformlyand steadily.

It will be furthermore noted that the aluminium carrying means maintainsthe aluminium metal content of the solution during the anodic period andalso gives greater conductivity to the circuit leading to the articlesuspendedj.

In my opinion the aluminium carrying memby Letters mium and mercury inwhich the cadmium zinc ratio is approximately 1 to 1 from an aqueousalkaline cyanide bath containing at least about 12 ounces of causticsoda per gallon of water and against corrosion comprising electricallydepositing thereon a ternary preponderatingly cadmium alloy of zinc,cadmium and mercury from an aqueous alkaline cyanide bath containing atleast about 12 ounces of caustic soda per gallon of water andcontaining-aluminum sulphate in solution in addition to the metal ionsof the alloy being deposited, and employing an alloy anode containingzinc, cadmium and mercury, the zincmercury ratio of the anode beingsubstantially 8 to 1 and the zinc-cadmium ratio thereof beingsubstantially 2 to 1.

3. A process for protecting a metal article against corrosion comprisingelectrically depositing thereon a ternary alloy containing zinc, cadmiumand mercury in which the cadmium-zinc ratio is approximately 1 to 1 froman aqueous alkaline cyanide bath containing at least about 12 ounces ofcaustic soda per gallon of water and containing aluminum sulphate insolution in addition to the metal ions of the alloy being deposited, andemploying an alloy anode containing zinc, cadmium and mercury, thezinc-mercury ratio being about 8 to 1 and the zinc-cadmium ratio thereofbeing from 2 to 1 to a ratio of 4 to 1.

4. A process for protecting a metal article against corrosion comprisingelectrically depositing thereon a ternary preponderatingly cadmium alloyof zinc, cadmium and mercury from an aqueous alkaline cyanide bathcontaining at least about 12 ounces of caustic soda per gallon of waterand containing aluminum sulphate in solution in addition to the metalions of the alloy, and employing an alloy anode containing zinc, cadmiumand mercury, the zinc-mercury ratio of the anode being substantially 8to 1 and the zinccadmium ratio thereof being from 2 to 1 to a ratio of 4to 1.

5. A process for protecting a metal article against corrosion comprisingelectrically depositing thereon a ternary alloy containing zinc, cadmiumand mercury in which the cadmium-zinc ratio is approximately 1 to 1 froman aqueous alkaline cyanidebath containing at least about 12 ounces ofcaustic soda per gallon of water and containing aluminum sulphate insolution in addition to. the metal ions of the alloy being deposited,and employing an alloy anode containing zinc, cadmium and mercury, thezinc-mercury ratio of the anode being about 8 to 1 and the zinccadmiumratio thereof being about 2 to l, the total combined content of the zincand cadmium in the bath in proportion to the mercury content beingsubstantially in the ratio of 2.5 to 1.

6. A process for protecting a metal article against corrosion comprisingelectrically depositing thereon a ternary preponderatingly cadmium alloyof zinc, cadmium and mercury from an aqueous alkaline cyanide bathcontaining at least about 12 ounces of caustic soda per gallon of watercontaining aluminum sulphate in solution in addition to the metal ionsof the alloy being deposited, and employing an alloy anode containingzinc, cadmium and mercury, the zincmercury ratio of the anode beingsubstantially 8 to 1 and the zinc-cadmium ratio thereof beingsubstantially 2 to 1, the total combined content of the zinc and cadmiumin the bath in proportion to the mercury content being substantially inthe ratio of 2.5 to 1.

SAMUEL THOMAS ROBERTS.

